This invention relates to a method and apparatus for time-period modulation (TPM) of a sinusoidal signal, such as a subcarrier for transmitting an audio signal, particularly in a stereophonic system of the compatible stereo-monaural type which combines left (L) and right (R) audio signals in the main channel as the sum (L+R) and in an auxiliary channel as the difference (L-R). A receiver equipped to detect the TPM difference (L-R) as well as the sum (L+R) can recover the left (L) and right (R) signals, while a receiver equipped to detect only the sum (L+R) will recover the complete audio program.
Although the present invention has immediat:e application transmitting stereophonic audio signals for television, it will be readily apparent to those skilled in the art of communications that the TPM technique will have other applications. However, because of its advantages over FM, which is the prior-art standard for stereophonic radio transmission, and which has been incorporated in three stereophonic transmission systems that have been proposed for television, the TPM technique will have its greatest application in stereophonic television.
For a summary of the proposed FM stereophonic television systems, and the performance objectives of any stereophonic television system, see a review by Ronald K. Jurgen, Senior Editor of IEEE Spectrum, September 1982, pp. 30-33 titled "Stereophonic sound for television" wherein "three transmission systems, vying for selection as the U.S. standard," are compared. The performance objectives include:
1. The L+R stereophonic system, as employed in FM stereo radio, should be used with signal-to-noise ratio and distortion equal to or better than the existing broadcast monophonic TV channel.
2. The dynamic range of the stereophonic system should not be limited by the dynamic characteristic of the monophonic signal.
3. A separate audio channel should be available for possible bilingual use.
4. Automatic switching should be available from a monaural mode to the stereo mode, as well as switching to bilingual channels, when more than monophonic sound is being received.
5. Auxiliary services should be accommodated (which implies providing auxiliary channels).
6. The use of compatible audio signal processors to improve the dynamic range and signal-to-noise ratio should be allowed.
While it is expected that all three FM stereophonic modulating systems proposed will meet at least the first two of these objectives, the present invention will meet all of the objectives, and some with improved performance. The most important is lower distortion and higher signal-to-noise ratio.
In frequency modulation of a carrier or a subcarrier, there is phase distortion at the high end of the frequency spectrum, particularly in high frequency, high level sound. The theory of FM modulation is that a carrier at a given center frequency is so modulated with an input audio signal that ideally the carrier frequency deviates in linear proportion to the audio signal. At the receiver, the FM signal is then detected to produce a voltage that is proportional to the amount of frequency deviation. The problem is that FM detector circuits of the integrating type actually measure time periods (T) of the FM cycles (except in the case of phase-lock loop discriminators).
While it is true that frequency is equal to 1/T, a measurement of the time period shift, .DELTA.T, of the frequency modulated carrier equal to 1/.DELTA.f is a "1 over X" function, which is known to be not linear. As a consequence, conversion of the audio signal to frequency shift of the carrier at the transmitter, and then converting the frequency shift to time period shift at the receiver, inherently introduces distortion. To avoid this distortion, it is possible to introduce a compensation function that is proportional to the measured time period, but the compensation circuit would be complex.
Another aspect of the distortion in an FM system: is the problem of a sudden change in the audio signal, such as that present in the attack of an instrument or voice, which should produce a corresponding shift in frequency. However, the reactance in the frequency modulator will not permit such sudden changes. The higher frequency is reached only over several cycles of the carrier during which there is an averaging effect from cycle to cycle. The problem can be compared to trying to make a sudden change in the RPM of a flywheel.
The TPM technique of the present invention allows a sudden change in cycle period to be made from one cycle to the next of the carrier in direct proportion to the amplitude of an audio signal. At the receiver, the period is measured to recover the audio signal. This not only avoids the flywheel effect in modulating, but also avoids the nonlinearity (1 over X function) of FM which results from the carrier frequency being changed in modulation, while the change in carrier period is detected in demodulation. This improvement is achieved in the present invention because the carrier period is changed in modulation, and the change in carrier period is detected in demodulation.